The invention disclosed and claimed herein generally pertains to a method for measuring change in T.sub.2 *, a parameter which is the combination of the spin-spin relaxation time and local magnetic field inhomogeneity effects, and parameters related thereto, in a subject of magnetic resonance imaging (MRI). More particularly, the invention pertains to a method of such type that employs a spiral scanning technique. This technique differs from conventional MRI imaging techniques in that in the acquisition of k-space data, data points are acquired along a spiral trajectory instead of a recti-linear trajectory.
The importance of measuring T.sub.2 * in biological systems is that it is known that the body's intrinsic contrast material, that of oxy and deoxy-hemaglobin has opposite effects on T.sub.2 *. It has been proposed that in regions of increased metabolic activity, the body responds by increasing the blood flow to that specific region. As a result of the increased blood flow, there is a greater concentration of oxy-hemaglobin. As oxy-hemaglobin is diamagnetic, increased concentrations of oxy-hemaglobin has the effect of reducing the local magnetic field susceptibility, thus increasing the local T.sub.2 *, resulting in increased signal intensity. Deoxy-hemaglobin is highly paramagnetic and has the effect of increasing the local magnetic field susceptibility and results in decreased T.sub.2 * (and subsequently, reduced signal intensity).
The increase in signal intensity as a result of increased perfusion or blood flow at the capillary level has been termed the BOLD effect or Blood Oxygenation Level Detection. This technique has been used to successfully detect metabolic activity in the brain without the use of exogeneous contrast agents. The use of BOLD allows repeated studies without the problems of having to wait for the body to purge itself of the exogeneous contrast agent.
In studies in the brain, there is less motion-related artifacts that tend to prevent accurate assessment of local perfusion. In the heart, cardiac and respiratory motion complicates the use of signal intensity changes to measure perfusion as partial volume effects, since motion of the excited region of tissue into and out of the region of interest would tend to alter the image signal intensity. Changes in the heart rate will also result in different degrees of signal recovery, changing image signal intensity from heart beat to heart beat.
As perfusion effects alter the tissue T.sub.2 *, measurement of this parameter would provide a better method for assessing perfusion that is less sensitive to motion-related artifacts. As T.sub.2 * is a measure of the rate of decay of signal, it suffices to measure the signal change at two points in time. The absolute intensity may vary from heart beat to heart beat as the heart rate changes, but the rate of signal decay will remain unchanged. It is well known to those skilled in the art that the image signal intensity is given as: EQU S(t)=S.sub.0 exp (-t/T.sub.2 *) Equation 1
where S.sub.0 is a function of proton density and other pulse sequence parameters. In order to determine the T.sub.2 * value, we require two measurements of the signal intensity, each at a different time point. The T.sub.2 * value can be determined by dividing the signal intensity at the two time points such that: EQU S(TE.sub.1)=S.sub.0 exp (-TE.sub.1 /T.sub.2 *) EQU S(TE.sub.2)=S.sub.0 exp (-TE.sub.2 /T.sub.2 *) Equation 2
where TE.sub.1 and TE.sub.2 are the two different echo times. Dividing the signals at these two time points gives EQU exp((TE.sub.2 -TE.sub.1)/T.sub.2 *)=S(TE.sub.1)/S(TE.sub.2)Equation 3
taking the natural logarithm on both sides and rearranging gives EQU T.sub.2 *=(TE.sub.2 -TE.sub.1)/1n(S(TE.sub.1)/S(TE.sub.2)) Equation 4
By measuring the tissue T.sub.2 * in a pre-stressed or activity state, and again in a post-stressed or during some other activity, changes in T.sub.2 * as a result of metabolic activity and perfusion can be assessed. For example, T.sub.2 * could be measured in a subject at rest, and then measured later during physical or pharmacologically induced exertion. This technique would be less sensitive to errors as previously elaborated than using changes in the absolute signal intensity levels as an indicator of perfusion.